This invention generally relates to valve plates for use in slide gate valves for controlling a flow of molten metal, and is specifically concerned with a valve plate that is resistant to cracks caused from thermomechanical stresses.
Slide gate valves are commonly used to control a flow of molten metal in steel making and other metallurgical processes. Such valves generally comprise a support frame, an upper stationary valve plate having an orifice in registry with a tundish or ladle nozzle for conducting a flow of molten metal, and a throttle plate likewise having a metal conducting orifice that is slidably movable under the stationary valve plate. In slide gate valves used in conjunction with continuous casting molds, a lower stationary valve plate is provided beneath the movable throttle plate which likewise has a flow conducting orifice that is substantially aligned with the orifice of the upper stationary plate. The rate of flow of molten metal is dependent upon the degree of overlap of the orifice of the slidably movable throttle plate with the orifice of the upper stationary plate. The movable throttle plate is usually longer than the stationary throttle plate in order to give it the capacity of throttling the flow of molten metal from both the front and back edges of its own orifice, as well as the ability to shut off the flow altogether by bringing its orifice completely outside of any overlap with the orifices of the stationary plate. Typically, the throttle plate is slidably manipulated between the stationary plates by means of a hydraulic linkage.
The throttle plate and the stationary plate are mounted in respectively a lower indentation and an upper indentation, each of these plates resting in an indentation through a surface that becomes its support surface and cooperating with the other plate through a surface that becomes its sliding or working surface.
Both the throttle plate and the stationary plates of such slide gate valves are formed from heat and erosion resistance refractory materials, such as aluminum oxide, alumina-carbon, zirconium oxide. However, despite the heat and erosion resistance of such refractory materials, the severe thermomechanical stresses that they are subjected to ultimately cause some degree of cracking to occur. For example, in steel making, each valve plate is subjected to temperatures of approximately 1600xc2x0 C. in the area immediately surrounding its flow-conducting orifice, while its exterior edges are experiencing only ambient temperature. The resulting large thermal gradient creates large amounts of thermomechanical stress as the area of each plate immediately surrounding its orifice expands at a substantially greater rate than the balance of the plate. These stresses cause cracks to form which radiate outwardly from the orifice of the plate. If nothing is done to contain the spread of these cracks, they can extend all the way to the outer edges of the plate, causing it to break.
To prevent the spreading of such cracks and the consequent breakage of the valve plates, various solutions have been developed in the prior art. In a first attempt, improved clamping mechanisms have been designed. The purpose of these mechanisms is to apply sufficient pressure around the perimeter of the plate so that cracks emanating from the orifice do not spread to the edges of the plate. One such mechanism comprises a frame having screw-operated wedges which engage corners of the plate that have been truncated in an angle that is complementary to the angle of the wedges. Such a system is disclosed in the document DE-C2-3,522,134. While such frame and wedge-type clamping mechanisms constitute an advance, the inventors have noted some shortcomings with this design that prevent it from achieving its full, crack-retarding potential. Generally, the clamping forces are not uniformly focused where the maximum amount of cracking occurs, i.e., in the vicinity of the orifice where the greatest amount of thermomechanical stresses are present. Moreover, the applicants have observed that, generally, the angular orientation of the truncated corners in such plates does not optimally prevent the spreading of cracks, as previously thought. Such non-optimality results from the fact that crack formation is not uniformly distributed 360xc2x0 around the orifice, but instead is biased along the longitudinal center line of all valve plates whether stationary or movable. Such an asymmetrical distribution of cracks around the plate orifices is believed to occur as a result of the longitudinal sliding action of the throttle plate across the faces of the stationary plates.
U.S. Pat. No. 5,626,164 discloses a crack resistant valve plate; the shape of said plate being designed to prevent the formation and spreading of cracks therein. This plate has an axis, and an orifice for conducting molten metal that is positioned along said axis, and truncated corners for focusing a clamping force toward said axis in the vicinity of said orifice, wherein each of said truncated corners is orthogonal to a line extending between a tangent point to said orifice, across said axis, and through an intersection of lines drawn parallel to converging plate edges that are spaced from said edges a distance equal to one-half of a width of said orifice.
In the document WO-A1-98/05451, there is disclosed a variant of this solution wherein the angles between the lateral faces of the plate are defined so as to extend the lifetime of the plate.
While U.S. Pat. No. 5,626,164 constitutes already a markedly clear advance over the previously known solution, applicants have tried to still optimize the plate shape. Clearly, there is a need for a valve plate whose shape optimally focus the clamping forces in the most crack-prone areas of the plate in order to maximally retard the lengthening of any such cracks. Ideally, the corners should have a length sufficient to avoid the production of unwanted localized mechanical stresses in the corners.
FIG. 2 of the document DE-A1-195 31 353 discloses a slide gate plate according to the preamble of claim 1. The edges, which are the closest to the pouring hole, are slightly inclined with respect to the rectangle elongation direction. It has been determined that with such an orientation of the edges, high tensile stress will develop around the pouring hole and could, in use, result in cracks radiating from the orifice towards the plate sides parallel to the rectangle elongation direction.
Generally speaking, the invention is a crack resistant valve plate assembly for use in a slide gate valve that overcomes or at least ameliorates the disadvantages associated with the prior art or that at least equals the performance of the plate disclosed in U.S. Pat. No. 5,626,164.
The invention relates thus to a refractory plate for a slide gate valve which may be circumscribed by an elongated rectangle R having two sides parallel to the direction of its elongation. The rectangle R has a longitudinal axis, which is defined as its longest symmetry axis and which will coincide with the preferential sliding trajectory of the plate. It is however to be clearly understood that this concept of preferential sliding trajectory is an intrinsic characteristic of the plate according to the invention and that this plate may be slid in a gate valve according to a direction which is not the optimal or preferential one.
The Plate has an orifice, that is, the pouring hole, for conducting molten metal. Most often said orifice is circular, more generally, it is circumscribed by a circle C of diameter xe2x96xa1. The orifice is positioned eccentrically in the middle between the parallel sides of the rectangle R.
For construction purpose, the rectangle R is divided into four quadrants by two perpendicular lines intersecting at the center of the circle C, one of these lines extending in the middle between the parallel sides of the rectangle R. Each quadrant has intersecting diagonals: diagonals D1, D3, D5, D7 joining the center of the circle C to the corners of the rectangle R and diagonals D2, D4, D6 and D8 joining adjacent intersections of the perpendicular lines intersecting at the center of the circle C with the sides of the rectangle R.
The pouring hole is offset along the longitudinal axis so that throttling may be effected on a longer area. The pouring hole may also be slightly offset along an axis perpendicular to the longitudinal axis.
The plate has angularly oriented edges, figuring the truncated corners of the rectangle R, for focusing clamping forces toward the vicinity of the orifice and toward the throttling area to prevent the formation and spreading of cracks therein.
According to the invention, at least a portion of the edges are defined as follows:
a) the edges farthest from the pouring hole (thus, closest to the throttling area) deviate at maximum 5xc2x0 from the direction of the diagonals which do not intersect the respective corner; and
b) the edges closest to the pouring hole (thus farthest from the throttling area) deviate at maximum 5xc2x0 from one of the following direction
(i) the direction perpendicular to the diagonal intersecting the respective corner;
(ii) the direction of the other diagonal of the respective quadrant; or
(iii) a direction intermediate between the directions (i) and (ii).
Applicants have indeed determined that such a plate shape focus optimally the clamping force to two different areas of the plate. On the one hand, the throttling area is kept in compression, preventing thus the apparition of cracks in that region and on the other hand, the perimeter of the pouring hole is also kept in compression, preventing thus the spreading of cracks radiating from the pouring hole.
Applicants have observed that the newly designed plate is extremely advantageous. Firstly, far less cracks are observed. Secondly, even if they still occur, the cracks do not spread up to the plate edges, so that air ingression is markedly reduced. And thirdly, when the plate according to the invention is used in combination with an appropriate clamping device, the cracks, if any, only occur in acceptable area, that is, they do not occur in the throttling area or directly in the area between the pouring hole and the closest edges.
The plate may be symmetrical with respect to its longitudinal axis, but in the preferred embodiment, the plate is not symmetrical with respect to the longitudinal axis. Because of this asymmetry, the plate may only be mounted in one position in the upper indentation and in one position in the lower indentation so that the support surface of the plate becomes its sliding or working surface when the plate passes from one position to the other in case recycling of the plates is desirable.
The plate may have only four edges defined as above, but in order to avoid sharp angles, it may have more edges. In such a case, the supplemental edges may (or not) be parallel and/or perpendicular to the longitudinal axis.
It must be understood that according to the present invention, it is not mandatory that the plate be polygonal. On the contrary, in case a clamping band is used around the plate, such clamping band can apply localized mechanical stresses, which could turn into cracks, to the vertex defined by adjacent edges. Therefore, it is advantageous that the corner be rounded.
In the preferred embodiment, only a portions of the edges satisfy the above definition. More preferably, the balance of the edges are comprised of curves joining the said edges portions and most preferably of transition radius of the said edges.